1. Field of the Invention
This invention relates generally to sampling systems which remove a sample from a process line containing a flowing fluid and, more particularly, to sampling systems which remove a specified volume of fluid from a process line.
2. Prior Art
There are many types of devices which remove samples from process lines. R. Wilson Spencer's prior U.S. Pat. Nos. 4,651,574, 4,791,821, 4,800,761 and 4,879,915 are examples of such devices. In specialized situations, often having a specified volume, with each successive sample having the same specified volume. It is tedious and sometimes dangerous to measure a specific volume of fluid into a sample container using standard sampling systems. In the case of hazardous fluid sampling, it is usually desirable to obtain a sample having a very small volume in order to limit exposure to the substance as well as limit the amount of neutralizing substance which must be added to the sample for safe handling in the lab.
In order to meet the sampling criteria set out above, prior art samplers have used a pair of three-way valves which are connected by a sample cylinder. One valve is connected to the process line and a pressurized gas source, usually an inert gas such as nitrogen. The second valve is connected to the process line and to a vented sample container. The operational parts of the valves are connected through the sample cylinder by a rotatable shaft which allows the valves to be operated simultaneously. Thus, a flow is established from the process line through the sample cylinder and back to the process line. Manipulation of the valves shuts off the flow from the process line, trapping a sample in the cylinder. Further operation of the valves allows the sample to be ejected from the cylinder by the pressurized gas into the sample container. The container is then removed and taken to a lab for analysis (and neutralization, if hazardous).
Prior art samplers do not facilitate easy changing of sample volumes. For example, if the operator desires to change from a 100 cc sample volume to a 50 cc sample volume, the sample cylinder and shaft (which is connected through the cylinder) must be removed requiring complete disassembly of the sampling system. Further, as the desired sample volume decreases, it is difficult to find standard sample cylinders haVing such small volumes which also have room for the internal shaft connecting the two valves. In addition to the above problems, cleaning of the prior art systems is impossible without disassembly, and the shaft is contaminated during operation. Also, plugging of the sample passageways is possible due to the restrictions caused by the narrow clearance of the shaft within the cylinder, as well as shaft connections within the valves.